Communications systems, and specifically messaging and paging systems, typically communicate on a channel using signals having a specified radio frequency. In normal operation messaging systems utilize a scheduled channel protocol where the system and a selective messaging unit by virtue of operating on the system know a predetermined time where communication is possible. This scheduled channel protocol facilitates battery savings since the unit or most of the circuitry in the unit is inactive or asleep most of the time. These systems operate as long as the channel or signal can be reliably received by selective messaging units operating within the system. Where the signal strength of the channel received by the selective messaging unit falls below a sensitivity level, the selective messaging unit will lose that channel.
Subsequently, the selective messaging unit will typically attempt an energy search to identify one or more channels with an appropriate signal. This energy search will ordinarily include the original channel lost, or may include other additional channels. Energy searches require the receive circuitry of the selective messaging unit to be active and tuned to the proper frequency. While active, this circuitry tends to require large amounts of energy or power relative to that required in an inactive state, and this power must be supplied from a very limited energy source such as a battery.
After the operating channel is lost, the selective messaging unit must rely on an internal clock and its accuracy for a time reference that will usually vary from the system time reference. Thus the selective messaging unit must do an energy search relatively often to acquire or re-acquire an operating channel in a reasonable period of time. Though design trade-offs can be made between the frequency of searches and the time required to obtain a viable operating channel versus the limited energy source, user expectations normally mitigate in favor of finding a channel rapidly and battery life ends up suffering.
Clearly a need exists to more effectively allow a selective messaging unit to acquire an operating channel while minimizing battery usage.